1. Field of the Invention
The present invention relates to a wireless communication system and, more particularly, to a reliable uplink channel quality information (CQI) coding method for HS-DPCCH in HSDPA system for 3GPP.
2. Description of the Background Art
The UMTS (Universal Mobile Telecommunications System) is the third generation mobile communication system evolved from a GSM (Global System for Mobile Communications) and a European style mobile communication standard. It is intended to provide improved mobile communication services on the basis of a GSM core network (CN) and a Wideband Code Division Multiple Access (WCDMA) access technology.
For the purpose of making a standard for third generation mobile communication systems (IMT-2000 systems) based on evolved GSM core network and WCDMA radio access technology, a group of standard developing organizations including ETSI of Europe, ARIB/TTC of Japan, T1 of U.S., and TTA of Korea established the Third Generation Partnership Project (3GPP).
For the purpose of efficient management and technological development, five Technical Specification Groups (TSGs) are organized under 3GPP in consideration of network construction factors and their operations.
Each TSG is in charge of approving, developing and managing specifications related to a pertinent area. Among them, RAN (Radio Access Network) group has developed functions, requirements and interface specifications related to UE (User Equipment) and UMTS terrestrial radio access network (UTRAN) in order to set a new radio access network specification to the third generation mobile communication system.
The TSG-RAN group consists of one plenary group and four working groups.
WG1 (Working Group 1) has been developing specifications for a physical layer (Layer 1), and WG2 has been specifying functions of a data link layer (Layer 2) between UE and UTRAN. In addition, WG3 has been developing specifications for interfaces among Node Bs (the Node B is a kind of base station in the wireless communications), Radio Network Controllers (RNCs) and the core network. Lastly, WG4 has been discussing requirements for radio link performance and radio resource management.
FIG. 1 illustrates a structure of the UTRAN defined in 3GPP.
As depicted in FIG. 1, the UTRAN 110 includes at least one or more radio network sub-systems (RNSs) 120 and 130, and each RNS includes one RNC and at least one or more Node Bs. For example, Node B 122 is managed by RNC 121, and receives information transmitted from the physical layer of the UE 150 through an uplink channel and transmits a data to the UE 150 through a downlink channel.
Accordingly, the Node B is considered to work as an access point of the UTRAN from the UE point of view.
The RNCs 121 and 131 perform functions of allocation and management of radio resources of the UMTS and are connected to a suitable core network element depending on types of services provided to users.
For example, the RNCs 121 and 131 are connected to a mobile switching center (MSC) 141 for a circuit-switched communication such as a voice call service, and are connected to a SGSN (Serving GPRS Support Node) 142 for a packet switched communication such as a radio Internet service.
The RNC in charge of a direct management of the Node B is called a Control RNC (CRNC) and the CRNC manages common radio resources.
On the other hand, the RNC that manages dedicated radio resources for a specific UE is called a Serving RNC (SRNC). Basically, the CRNC and the SRNC can be co-located in the same physical node. However, if the UE has been moved to an area of a new RNC that is different from SRNC, the CRNC and the SRNC may be located at physically different places.
There is an interface that can operate as a communication path between various network elements. The interface between a Node B and a RNC is called a lub interface, and an interface between RNCs is called an lur interface. And an interface between the RNC and the core network is called an lu.
High Speed Data Packet Access (HSDPA) is standardization work within the 3GPP for realizing high speed, high-quality wireless data packet services. To support HSDPA, various advanced technologies such as Adaptive Modulation and Coding (AMC), Hybrid Automatic Repeat Request (HARQ), Fast Cell Selection (FCS), Multiple Input Multiple Out (MIMO), and etc. are introduced.
Well known are the benefits of adapting the transmission parameters in a wireless system to the changing channel conditions. The process of modifying the transmission parameters to compensate for the variations in channel condition is known as link adaptation (LA) and AMC is one of the link adaptation techniques. The principle of AMC is to change the modulation and coding scheme according to variations in the channel conditions, subject to system restrictions. That channel conditions can be estimated based on feedback from the UE. In a system with AMC, the UEs in favorable positions, i.e., close to the cell site, are typically assigned higher order modulation with higher code rate (e.g. 64 QAM with R=¾ Turbo Code), while UEs in unfavorable positions, i.e., close to the cell boundary, are assigned lower order modulation with lower code rate (e.g. QPSK with R=½ Turbo Code). The main benefits of AMC are the higher data rate available for UEs in favorable positions which in turn increases the average throughput of the cell and the reduced interference variation due to link adaptation based on variations in the modulation/coding scheme instead of variations in transmit power.
In conventional ARQ, ARQ process should be performed along up to the upper layer of the UE and the node B, while in the HSDPA, ARQ process is conducted within the physical layer. The key characteristic of the HARQ is to transmit the un-transmitted portion of the encoded block when the NACK (No Acknowledgement) is received from the receiver, which enables the receiver to combine each portion of received codewords into the new codewords with the lower coding rate so as to obtain much coding gain. Another feature of the n-channel HARQ is that a plurality of packets can be transmitted on n channels even when an ACK/NACK (Acknowledgement/No acknowledgement) is not received unlike in the typically Stop and Wait ARQ which allows the node B to transmit the next packet only when the ACK signal is received from the receiver or to retransmit the previous packet when the NACK signal is received. In other words, the node B of HSDPA can transmit a plurality of next packets successively even if it does not receive the ACK/NACK for the previous transmitted packet, thereby increasing channel usage efficiency. Combining AMC and HARQ leads to maximize transmission efficiency-AMC provides the coarse data rate selection, while HARQ provides fine data rate adjustment based on channel conditions.
FCS is conceptually similar to Site Selection Diversity Transmission (SSDT). Using FCS, the UE indicates the best cell which should serve it on the downlink, through uplink signaling. Thus while multiple cells may be members of the active set, only one of them transmits at a certain time, potentially decreasing interference and increasing system capacity. Determination of the best cell may not only be based on radio propagation conditions but also available resources such as power and code space for the cells in the active set.
MIMO is one of the diversity techniques based on the use of multiple downlink transmit/receiver antennas. MIMO processing employs multiple antennas at both the base station transmitter and terminal receiver, providing several advantages over transmit diversity techniques with multiple antennas only at the transmitter and over conventional single antenna systems.
Due to the introductions of these new schemes, new control signals are configured between the UE and the node B in HSDPA. HS-DPCCH is a modification to UL DPCCH for supporting HSDPA.
FIG. 2 shows a frame structure for uplink HS-DPCCH associated with HS-DSCH transmission. The HS-DPCCH carries uplink feedback signaling consisted of HARQ-ACK/NACK and channel-quality indicator (CQI). Each subframe of length 2 ms (3×2560 chips) consists of 3 slots, each of length 2560 chips. The HARQ-ACK/NACK is carried in the first slot of the HS-DPCCH subframe and the CQI is carried in the second and third slot of the HS-DPCCH subframe. There is at most one HS-DPCCH on each radio link and the HS-DPCCH can only exist together with an uplink DPCCH.
To support fast link adaptation, the UE is to provide node B with information about the downlink channel quality, i.e., CQI. Regarding the channel coding for HS-DPCCH CQI, a number of uplink CQI coding methods have been proposed and most proposals assume that the CQI is to be coded into 20 channel bits. The CQI coding methods are based on the Transmit Format Combination Indicator (TFCI) coding method of 3GPP specification. FIG. 3a shows a (16, 5) TFCI encoder, which is similar to the (32, 10) TFCI encoder in FIG. 3b except that five information bits are used so as to generate (16, 5) TFCI codeword. The basis sequences for (16, 5) TFCI code are shown in table 1a and the basis sequences for (32, 10) TFCI code are illustrated in table 1b.
Detailed methods of generating TFCI codeword are revisited below. First, (16, 5) TFCI encoding method is described. In table 1a, let the TFCI information bits a0, a1, a2, a3, a4, and Mi,n a basis sequence for n-th TFCI information bit. Then output codeword bits bi are given by
            b      i        =                            ∑                      n            =            0                    4                ⁢                              (                                          a                n                            ×                              M                                  i                  ,                  n                                                      )                    ⁢                                          ⁢          mod          ⁢                                          ⁢          2          ⁢                                          ⁢          where          ⁢                                          ⁢          i                    =      0        ,  1  ,  2  ,      …    ⁢                  ⁢    15  The output bits are denoted by bi, i=0, 1, 2, . . . 15.
In a similar manner, the generation of (32, 10) TFCI codeword can be defined. In table 1b, let the TFCI information bits a0, a1, a2, a3, a4, a5, a6, a7, a8, a9 and Mi,n a basis sequence for n-th TFCI information bit. Then output codeword bits bi are given by
            b      i        =                            ∑                      n            =            0                    9                ⁢                              (                                          a                n                            ×                              M                                  i                  ,                  n                                                      )                    ⁢                                          ⁢          mod          ⁢                                          ⁢          2          ⁢                                          ⁢          where          ⁢                                          ⁢          i                    =      0        ,  1  ,  2  ,      …    ⁢                  ⁢    31  The output bits are denoted by bi, i=0, 1, 2, . . . 31.
The basis sequences for (16, 5) TFCI in Table 1a are included in the basis sequences for (32, 10) TFCI in Table 1b if the information bits are limited to the first 5 bits and the some 16 output bits are selected from 32 output bits. The common part between two basis sequences is highlighted by shadow in table 1b. The CQI coding method is based on the conventional TFCI coding method. The CQI requires 5 information bits and 20 coded bits, i.e. (20, 5) CQI code. Therefore, the (16, 5) TFCI code and (32, 10) TFCI coding method should be modified to fit the required number of bits for CQI coding. The (16, 5) TFCI code should be extended to (20, 5) CQI code by adding each basis sequence by 4 bits. The (32, 10) TFCI code can be used to generate (20, 5) CQI code through two steps. First, the (32, 10) TFCI code should be expurgated to the (32, 5) modified TFCI code by deleting last 5 basis sequences. Hereinafter the (32, 5) modified TFCI code by deleting last 5 basis sequences is referred to (32, 5) expurgated TFCI code. Secondly, the (32, 5) expurgated TFCI code should be punctured and repeated to meet the (20, 5) CQI code. The basis sequences for the (32, 5) expurgated TFCI code are as follows in table 1c. The common part of basis sequences between (16, 5) TFCI code and (32, 5) expurgated TFCI code is shadowed. The table 1c also include the basis sequences for (16, 5) TFCI code, i.e. table 1a. It means that the generating method based on the (32, 10) TFCI code can be represented by another form of generating method based on the (16, 5) TFCI code, vice versa.
TABLE 1aiMi,0Mi,1Mi,2Mi,3Mi,4010001101001211001300101410101501101611101700011810011901011101101111001111210111130111114111111500001
TABLE 1biMi,0Mi,1Mi,2Mi,3Mi,4Mi,5Mi,6Mi,7Mi,8Mi,901000010000101000110002110001000130010011011410100100015011001001061110010100700010101108100101111090101011011101101010011110011010110121011010101130111011001141111011111151000111100160100111101171100111010180010110111191010110101200110110011211110110111220001110100231001111101240101111010251101111001260011110010271011111100280111111110291111111111300000010000310000111000
TABLE 1ciMi,0Mi,1Mi,2Mi,3Mi,40100001010002110003001004101005011006111007000108100109010101011010110011012101101301110141111015100011601001171100118001011910101200110121111012200011231001124010112511011260011127101112801111291111130000003100001
FIG. 4 illustrates an encoder for generating an extended (16, 5) TFCI code. In FIG. 4, (16, 5) TFCI code, is reused with each codeword extended with the four least reliable information bits for (20, 5) CQI code. This CQI coding scheme is designed so as to have the optimal minimum distance.
FIG. 5a illustrates an encoder for generating punctured (32, 5) expurgated TFCI code. In this CQI coding scheme, (32, 5) expurgated TFCI code with puncturing 12 symbols is proposed. The puncturing pattern and used basis sequences are as in FIG. 5b. 
However, (20, 5) CQI coding schemes using the extended (16, 5) TFCI code in FIG. 4 and the punctured (32, 5) expurgated TFCI code in FIG. 5 are equivalent to each other. That is because the resultant basis sequences based on the (16, 5) TFCI code are the same as the resultant punctured basis sequences based on the (32, 5) expurgated TFCI code after puncturing. The only difference is the order of codeword bits. However, since the difference of bit position doesn't have any effect on the coding performances and properties, both coding schemes of FIG. 4 and FIG. 5 are equivalent each other.
Since the (20, 5) CQI coding scheme based on the (16, 5) TFCI code can be expressed as that based on the (32, 5) expurgated TFCI code, vice versa, the extended (16, 5) TFCI code and the punctured (32, 5) expurgated TFCI code are commonly expressed as the basis sequences in table 2. It means that the (20, 5) CQI coding scheme based on both the (16, 5) TFCI and (32, 5) expurgated TFCI code is to decide what the basis sequence pattern is in the blank in table 2. Hereinafter, the basis sequence part which is the same as 3GPP technical specifications will be omitted for convenience.
TABLE 2iMi,0Mi,1Mi,2Mi,3Mi,4 010001..................1411111150000116To be filled with the extended patterns in the embodiments17181920
FIG. 6 illustrates another encoder for generating extended (16, 5) TFCI code. In order to extend from (16, 5) to (20, 5), the basis sequence is extended and the extended parts are filled as in table 3.
TABLE 3iMi,0Mi,1Mi,2Mi,3Mi,4 010001..................15000011600001170000118000011900010
Here Mi,4 is the most significant bit (MOB). This arrangement gives significant extra protection to the MOB, and a little more robustness to the next most significant bit.
The conventional CQI coding schemes and their performances are varied according to the extended parts of basis sequence table. In this approach, to select optimum CQI coding scheme means just to find optimum extended part of the basis sequence table.
The above CQI coding schemes are developed in consideration of BER performance and unequal error protection (RMS error reduction) but system throughput. However, the coding schemes have tradeoffs between BER and unequal error protection. In other words, in view of the BER performance the first and second CQI coding schemes are superior to that of the third one. On the other hand, in view of the unequal error protection the third CQI coding scheme is superior to those of the first and second ones.
However, since HSDPA system has been designed in order to increase the system throughput, it is desirable to use the system throughput as one of the criteria in order to select optimum CQI coding scheme.